Capacitors are widely used devices for storing electrical energy. Among the various types of capacitors are electrochemical capacitors and electrolytic capacitors.
Electrolytic capacitors consist of series combinations of two capacitors (e.g. foil electrodes or plates), separated by an electrolyte, and between which a dielectric oxide film is formed adjacent to the surface of one or both of the electrodes.
Electrochemical capacitors consist of a series combination of at least two capacitors separated by an electrolyte. Each capacitor is formed by electrochemical processes (double layer charge storage or Faradaic pseudocapacitance charge storage) across an interface, such as the interface between an electrolyte and an electrode. Such capacitors rely on charge accumulation at the interface in order to store energy. Such capacitors do not rely on a dielectric oxide film for charge storage.
It is well known to produce electrochemical capacitors with electrodes made of carbon materials, with an electrolyte between the electrodes.
An important parameter in selection of capacitors is energy density. The energy density of a capacitor is the amount of energy stored per unit volume or mass of the capacitor. Among the desirable characteristics of capacitors is high energy density, since high energy density capacitors result in decreased capacitor mass and volume required for a given task.
Various approaches have been investigated to increase the energy density of electrochemical capacitors, while still allowing them to provide high power performance. One such approach has been to use non-aqueous electrolytes, which can increase operating voltage and thus energy density. However, non-aqueous electrolytes have low conductivity compared to aqueous electrolytes and thus lower power performance. Further, such electrolytes can be expensive, unstable, and prone to contamination by water and/or air.
From the foregoing is clear that a need exists for improved capacitor having large energy density and long service life.